1. Field of the Invention
This invention relates to a developer supply device for maintaining constant the developing effect of a two-component developer for image formation apparatus which consists of a mixture of toner and carrier.
2. Description of the Prior Art
Generally, in an image formation apparatus for forming an image by a magnetic latent image or an electrostatic latent image, use is made of a two-component developer consisting of a mixture of toner and carrier. It is well known that the weight admixture ratio of toner and carrier in such developer is a very important factor from the viewpoint of the developing effect. For example, when the proportion of toner to carrier (hereinafter simply referred to as the toner density) is low, namely, when the density of the toner in the developer is low, the density of the image after developed becomes thin. Conversely, when the proportion of toner is too high, the density of the developed image becomes too thick and also fog is increased. Accordingly, in order that images of preferable color tone may be continuously obtained, it is necessary to render the density of the toner in the developer into a proper level and moreover to maintain the level constant always during development.
However, in the conventional developer supply devices, although the developer dealt with therein has been a two-component developer, the developer supplied as a replenishing agent has been toner alone or a developer consisting of toner and carrier mixed therewith at a very small proportion, and the conventional developer supply devices have not been such that supply of carrier is effected separately from the supply of toner. Accordingly, there has been adopted a method whereby toner alone or toner with a small amount of carrier mixed therewith is supplied to compensate for the reduction in toner density which takes place in accordance with toner consumption, and this has led to a disadvantage that even if the toner density can be maintained substantially constant, the amount of the developer (mixture of toner and carrier) within the developing device varies. Also, in the conventional developer supply devices, the supply of carrier is not independently effected and therefore, when toner alone is supplied, a variation in volume of the developer has resulted from the consumption of carrier (actually, carrier is also consumed little by little as will later be described), and in the case of the supply of toner with carrier mixed therewith, there has been a disadvantage that if a balance with the consumed carrier is not kept, a variation in volume of the developer may also occur.
Such a variation in volume of the developer within the developing device cannot be visually confirmed because the developing device is usually mounted deep in the dark interior of an electrophotographic apparatus body and heretofore, it has been as much as one can do to judge such variation by referring to the density of the image on the resultant copy paper or to detect it only when the developer has become less than a predetermined amount. Accordingly, the fact that any change for increase or decrease in amount of the developer from a preferable amount cannot be known has necessarily resulted in adverse effects such as instability of image density attributable to the variation in amount of the developer, increased fog, reduced sharpness of the image portion, irregularity of development, etc., and has also adversely affected the stirring and conveyance of the developer.
Therefore, the assignee of the present invention has proposed, in Japanese Laid-open Patent Application No. 48254/1979, a developer supply device which supplies toner and carrier separately. The present invention intends to improve the carrier supply method in such a developer supply device.